The present invention generally concerns acoustic wave devices and methods for their fabrication; and more particularly, in various representative and exemplary embodiments, to flexural plate wave micro-pumps, micro-filters and sensor elements having epitaxial PZT piezoelectric membranes.
For many years, attempts have been made to grow various monolithic thin films on foreign substrates such as silicon. To achieve optimal characteristics of various monolithic layers, however, a monocrystalline film of high crystalline quality is typically desired. Attempts have been made, for example, to grow various monocrystalline layers on a substrate such as germanium, silicon, and various insulators. These attempts have generally been unsuccessful due in part to lattice mismatches between the host crystal and the grown crystal causing the resulting layer of monocrystalline material to be of relatively low crystalline quality.
Acoustic plate wave devices have several applications in the microelectronics industry. For example, flexural plate wave devices may be used to perform active or passive signal processing functions suitable for delay lines, attenuators, phase shifters, filters, amplifiers, oscillators, mixers, limiters, and the like. Such devices are often connected to other microelectronics components such as integrated circuits and RF generators to form assemblies for telecommunication, digital processing as well as other applications.
Plate wave devices generally include a transducer coupled to piezoelectric material that converts an electronic signal received from the transducer to an acoustic plate wave. The plate modes generally consist of acoustic wave energy reflected back and forth between the faces of the piezoelectric plate while propagating along the length of the crystal. Flexural plate wave devices are often fabricated by forming the transducer on the surface of a piezoelectric material or over a substrate, which itself may or may not be piezoelectric.
Attempts have also been made to grow thin-films of piezoelectric material over a semiconductor substrate. Formation of such films on semiconductor substrates is desirable because it allows for the integration of acoustic wave devices with other microelectronics devices on a substantially unitary substrate. However, thin films of piezoelectric material formed on semiconductor substrates are of generally lesser quality than bulk piezoelectric material because lattice mismatches between the host crystal, or semiconductor substrate, and the grown crystal, or piezoelectric material, typically cause the grown thin film of piezoelectric material to be of relatively low crystalline quality. Furthermore, such thin films of piezoelectric material must generally be chosen from a set of materials that are generally compatible with the semiconductor substrate.
Moreover, the desirable characteristics of acoustic plate wave devices increase as the crystallinity of the piezoelectric film increases. For example, the electromechanical coupling coefficient and the piezoelectric coefficient of a piezoelectric material in monocrystalline form are typically higher than those of the same material in polycrystalline or amorphous form. If a large area thin film of high quality monocrystalline piezoelectric material were available at relatively low cost, a variety of flexural plate wave devices could advantageously be fabricated using such a film at relatively low cost compared to that of fabricating such devices on a bulk wafer of the piezoelectric material or on an epitaxial film of such material on, for example, a sapphire substrate. In addition, if thin films of high quality monocrystalline piezoelectric material and compound semiconductor material could be realized on a bulk wafer, such as a silicon wafer, an integrated device structure could be achieved that advantageously uses the properties of both the compound semiconductor material and the piezoelectric material. Modular technologies such as low temperature co-fired ceramic (LTCC) technologies can be adapted to combine diverse substrates, however the overall size of such devices generally may not be readily reduced, as compared to the aggregate size of discrete devices, due inter alia to interconnection requirements for wire bonding and the like. Accordingly, a need exists for a flexural plate wave system formed from high-quality, highly-ordered, monocrystalline piezoelectric material.
Acoustic waves (AW) in piezoelectric structures have been previously used to measure liquids in contact with piezoelectric structures. See, for example, U.S. Pat. No. 4,378,168 to Kuisma which discloses a piezoelectric device having spaced input and output electrodes on a surface for generating a surface acoustic wave (SAW) for detecting the presence of humidity between the electrodes as a function of signal attenuation.
The use of devices that employ Rayleigh waves (i.e., SAWs) to sense mass changes at solid/gas interfaces is also known; however, SAWs are generally impractical for use in sensor applications because Rayleigh waves typically do not propagate efficiently at solid-liquid interfaces. For example, Rayleigh waves generally have a surface-normal component of particle displacement which typically generates compressive waves in a liquid, thereby leading to substantial signal attenuation.
U.S. Pat. No. 4,691,714 to Wong et al. generally discloses the use of bulk acoustic waves within a piezoelectric structure to measure viscosity of a liquid in contact therewith. The viscosity of the liquid is described as a function of the amplitude of the transmitted bulk acoustic signal while the temperature of the liquid (i.e., the temperature of the sensing transducer) is a function of the phase of the SAW propagated in the device.
U.S. Pat. No. 5,117,146 to Martin et al. discloses solid-state acoustic sensors for monitoring conditions at a surface immersed in a liquid by placement of inter-digitated input and output transducers on a piezoelectric substrate with the propagation of acoustic plate modes therebetween, but does not enable, disclose or otherwise suggest structure and/or means for the fabrication of such a device from high-quality, highly-ordered, monocrystalline material.
U.S. Pat. No. 6,232,139 and WO Patent WO0171336 disclose the fabrication of plate wave devices employing sputter deposited AIN and ZnO layers.
More recent work has focused on employing polycrystalline lead zirconate titanate, which has a somewhat higher piezoelectric coefficient. All of this work, however, has been generally limited to polycrystalline piezoelectric films.
In various representative aspects, the present invention provides a system and method for providing flexural plate wave micropumps, microfilters, and/or sensor components comprising, for example, epitaxial PZT as a piezoelectric membrane. An exemplary system and method for providing an acoustic plate wave apparatus is disclosed as comprising inter alia: a monocrystalline silicon substrate; an amorphous oxide material layer; a monocrystalline perovskite oxide material layer; a monocrystalline piezoelectric material layer; and a flexural plate wave component having an input interdigitated transducer, an output interdigitated transducer and an optional support layer. Deposition or removal of material on or from an absorptive thin film sensor surface, or changes in the mechanical properties of the thin film upon making contact with various chemical species, or changes in the electrical characteristics of a solvent solution exposed to the thin film generally operate to produce measurable perturbations in the vector quantities (e.g., velocity, etc.) and scalar quantities (e.g., attenuation, etc.) of the acoustic plate modes.
When monolithically integrated with silicon or compound semiconductor devices, a representative design is disclosed as comprising highly miniaturized, self-contained sensing and analysis components with remote wireless monitoring capabilities; however, the disclosed system and method may be readily and more generally adapted for use in any plate wave driven system or application. For example, the present invention may embody a device and/or method for providing integrated pumping and/or valving systems for use in microfluidic, sensor, chromatographic or other partitioning applications. Additional advantages of the present invention will be set forth in the Detailed Description which follows and may be obvious from the Detailed Description or may be learned by practice of exemplary embodiments of the invention. Still other advantages of the invention may be realized by means of any of the instrumentalities, methods or combinations particularly pointed out in the Claims.